A progressive addition lens which is used as a glasses lens for correction for presbyopia of a user's vision has been developed. For example, a progressive addition power lens has a region (distance portion), which has a refractive power for viewing a distance field, arranged between an upper part and a center part of the lens, a region (near portion), which has a refractive power for viewing a near field, arranged at a lower part of the lens, and a region (intermediate portion) which smoothly connects the distance portion and the near portion each of which has different refractive power. In the intermediate portion, the refractive power is continuously changed from the refractive power of the distance portion to the refractive power of the near portion.
By using such a progressive addition lens as a glasses lens, a state of difficulty in focusing on the near field due to the presbyopia is improved. Further, when a vision line is moved between the distance field and the near field, such a configuration makes it possible to move the vision line with less incongruous across a wide range between the distance field and the near field.
However, in the progressive addition lens, since different regions having different refractive powers are arranged on a single lens, when a user sees an object through the lens, a jumping and a warping of images of the object is generated and therefore comfortableness in use is deteriorated. A progressive addition lens which improves such a jumping and a warping of images is disclosed for example in Patent Literature 1.
In Patent Literature 1, a surface refractive power in a horizontal direction of the lens is larger than a surface refractive power in a vertical direction in an object-side surface of the progressive addition lens. With such a configuration, a fluctuation of angle of the vision line passing through the lens is suppressed when the vision line is moved in the horizontal direction. As a result, various aberrations of the object seen through the lens are reduced and the jumping of images is improved.
Further, a progressive addition lens which reduces the jumping and the warping of images is disclosed for example in Patent Literature 2.
In Patent Literature 2, an arrangement of a lens surface of the progressive addition lens is focused. Conventionally, a progressive refractive surface having a progressive refractive effect is arranged on the object-side surface (outer surface), however in Patent Literature 2, a so-called inner surface progressive addition lens is disclosed in which the progressive refractive surface is arranged on the eyeball-side surface and the object-side surface is formed in a spherical surface.
In the inner surface progressive addition lens, by providing a progressive refractive surface (a surface in which a curvature is changed) in which regions having different refractive powers (distance portion and near portion) are formed as an inner surface and providing a surface (a surface in which a curvature is constant) in which a region having a constant refractive power is formed as the object-side surface, “shape factor” which is one of factors determining a magnification of the glasses lens is set to be constant and therefore magnification difference generated in the glasses lens can be reduced.
Specifically, in a lens 100 shown in FIG. 11, the magnification of the glasses lens (S. M.) is represented by a Formula 1 by using a shape factor (Ms) and a power factor (Mp) as below.S.M.=Ms·Mp  Formula 1
Further, the Ms is represented by a Formula 2 and the Mp is represented by a Formula 3 as below.
                    [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          1                ]                                                            Ms        =                  1                      1            -                                          D                ⁢                                                                  ⁢                                  1                  ·                  t                                            n                                                          Formula        ⁢                                  ⁢        2                                [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          2                ]                                                            Mp        =                  1                      1            -                          L              ·              P                                                          Formula        ⁢                                  ⁢        3            
Here, D1 denotes a base curve of an object-side surface 200, t denotes a thickness of the lens center, n denotes a refractive index of the lens, L denotes a distance between an apex (inner apex) of an eyeball-side surface 300 and an eyeball E (specifically, a corneal apex), and P denotes a power at the inner apex.
It is obvious from the Formula 1 that the magnification of the glasses lens is changed in accordance with the shape factor and the power factor.
Here, in the Formula 2, the D1 in the shape factor is the base curve of the object-side surface, and when the progressive refractive surface is provided as the object-side surface, since the refractive power or the base curve in the distance portion and that in the near portion are different, the D1 is not to be constant and therefore the D1 is fluctuated. Accordingly, the shape factor is fluctuated.
Further, in the power factor, since the P shown in the Formula 3 is unambiguously determined by the power (distance power, near power, addition power and the like) added to the lens, the P cannot be set freely.
However, as disclosed in Patent Literature 2, by providing the progressive refractive surface as the eyeball-side surface and forming the object-side surface in the spherical surface, the base curve is not changed but is to be constant. Accordingly, by setting the shape factor to be constant by forming the object-side surface in the spherical surface, the fluctuation of the magnification due to the shape factor disappears with respect to the difference of the magnification of the glasses lens (S. M.), and therefore the difference of the magnification can be reduced.